1. Field of the Invention
The present invention relates generally to a three-dimensional fabric suitable as the frame member of a composite. More particularly, this invention relates to a three-dimensional fabric which has layers of bias yarns which are obliquely arranged.
2 Description of the Related Art
Composite materials having a three-dimensional fabric as a frame member, are expected to be widely used as structural materials for rockets, aircraft, automobiles, marine vessels and buildings. A composite material generally includes three kinds of threads or yarns, extending in the X, Y and Z axial directions. The frame member is impregnated with a resin or another inorganic substance. While this type of composite-material shows sufficient strength to tensile, compressive, and bending loads along the X, Y and Z directions, it is still deformable by forces along the oblique direction with respect to each axial direction.
One attempt to rectify this undesirable characteristic, was described in Japanese Unexamined Patent Publication No. 1-292162, Japanese Unexamined Patent Publication No. 3-76845 and WO 90/14454. These publications describe five-axis three-dimensional fabrics, with bias yarns that are angularly arranged with respect to the lengthwise direction of the three-dimensional fabric. These bias yarns are in addition to yarns extending in the X, Y and Z axial directions.
As shown in FIG. 13, the conventional five-axis three-dimensional fabric comprises a warp layer consisting of multiple lines of a warp z, arranged parallel to the lengthwise direction of the fabric. Bias-yarn layers including two types of bias yarns B.sub.1 and B.sub.2 are angularly arranged with respect to the lengthwise direction of the fabric, in a plane parallel to the warp layer. A weft layer consisting of lines of a weft x are arranged in the width direction of the fabric, perpendicularly to the warp z. The five-axis three-dimensional fabric further includes a number of vertical yarns y arranged in the thickness or transverse direction of the fabric, perpendicularly to the warp z.
Each bias-yarn layer is constituted of a pair of two adjoining, upper and lower yarn layers. The arrangement of the bias yarns B.sub.1 and B.sub.2 is identical in any pair of upper and lower yarn layers. More specifically, the bias yarns B.sub.1 of the upper bias-yarn layer are arranged always at an inclination of +.THETA. with respect to the warp yarns z, while the bias yarns B.sub.2 of the lower bias-yarn layer are arranged always at an inclination of -.THETA. with respect to the warp yarns z.
In producing a composite having a three-dimensional fabric as its frame member, at the time the three-dimensional fabric is impregnated with a resin to be hardened, the composite contracts due to the hardening of the resin and yarn deformation. If the fibers of the three-dimensional fabric were carbon fibers or the like, which have a small coefficient of linear expansion and high rigidity, the contraction does not occur in the axial directions of the yarns, but rather occurs only in the directions perpendicular to the yarn axes. In this case, the contractional directions of the warp layer, weft layer and bias-yarn layers are different from one another. Depending on the fabrication conditions, such as the laminating order of the individual layers and the number of layers, the composite would bend or twist after the hardening of the resin.
In the conventional three-dimensional fabric, the first and second bias yarns (B.sub.1, B.sub.2) in any pair of the bias yarns, are vertically arranged in the same manner. If a composite having the conventional three-dimensional fabric as its frame member is produced, the residual deformations of the individual yarn layers would be added together at the time of resin impregnation and hardening. Such residual deformations will warp or twist the plate-like composite, thus reducing the accuracy in sizing the composite.